In a principal aspect, the present invention relates to a high-performance carburized gear steel that can improve the performance of rotorcraft power transmissions, due to a unique and useful combination of surface hardness and core toughness. The U.S. Navy estimates that a 20% increase in gear durability would provide an annual cost saving of $17 million to the Defense Logistics Agency. However, the rotorcraft industry has not adopted a new gear steel for over twenty years, and instead focused on surface processing optimizations such as laser-peening, super-finishing, and directional forging. Such processes are providing diminishing returns in durability improvements. The subject invention provides a solution complementary to process enhancements and enables high-performance gears having reduced size and weight which are capable of transmitting more power at increased operating temperatures.
Carburized X53 (U.S. Pat. No. 4,157,258) is the incumbent material in rotorcraft transmissions. Compared to X53, the subject invention places an emphasis on increasing the case strength and the core fracture toughness, as well as increasing the thermal stability up to 450° C. to provide hot hardness in high-temperature excursions. U.S. Pat. No. 6,464,801 also discloses case-hardened steels. However, the embodiment A1 of U.S. Pat. No. 6,464,801 shows limited surface-hardness, i.e., Rockwell C scale hardness (HRC) of 60-62. Another embodiment of U.S. Pat. No. 6,464,801, steel C3, shows a greater surface hardness of 69 HRC, but the core of this steel lacks toughness. To be usable as a gear, the core fracture toughness of the steel must exceed 50 ksi√in. Thus, there has developed a need for a carburized gear steel with a surface hardness of HRC of at least about 62-64 at a usable core toughness exceeding 50 ksi√in.